Any animal that moves through a fluid leaves behind a tell-tale trail of spinning vortices that can be interpreted to reveal the aerodynamic forces that keep them aloft. While several species of bird have been put through their paces in wind tunnels to reveal the intricate vortex patterns and lift forces that keep them airborne, less is known about the vorticity trails left by bats. Tatjana Hubel and her colleagues from Brown University, USA, flew four lesser dog-faced fruit bats (Cynopterus brachyotis) at a low (∼5 m s–1) and medium speed (∼6.7 m s–1) in a wind tunnel while filming the animals' wing beat patterns and wake structures to find out how the vortices in their wakes correlated with their wing beats and how much the wake and wing beat patterns vary between individuals (p. 3427).

The team found a distinctive wake pattern containing four different vortices at both speeds that was dominated by a strong wingtip vortex that developed during the downstroke. Persisting almost to the end of the upstroke, the vortex closely tracked the position of the wingtip during the downstroke, but shifted slightly towards the bat's body during the upstroke. At the same time as the wingtip vortex appeared, a second, clockwise spinning vortex was generated next to the body during the first half of the downstroke and two additional vortices were generated just above the wingtip during the final third of the upstroke.

The team also compared the wing beat pattern of each bat with that of the others, and saw that each animal had its own distinctive wing beat signature when changing speed. Also, most individuals varied the wing beat amplitude, body-to-wingtip distance and stroke plane angle as they increased their speed.

Having revealed the lesser dog-faced fruit bats' wake vorticity patterns, the team are keen to compare it with the vorticity patterns of other bats that hunt insects, hover at flowers and even migrate, to find out how they adapt their wing beat and lift patterns to radically different lifestyles.

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